Function-oriented Knowledge Base \ Electronic paper \ Improve image quality of paper-like display
Microencapsulation of electrophoretic composition improves electrophoretic display stability
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Introduction Electrophoretic displays are paper-like non-emissive
displays, which utilize the phenomenon of electrophoresis
to achieve a contrasting image. A conventional electrophoretic
display is bistable; its state persists after the activating
electric field is removed. The state persists via the residual
charge on the electrodes and van der Waals interactions between
the electrophoretic particles and the walls of the electrophoretic
cell. However, conventional electrophoretic displays are insufficiently
stable because they are affected by agglomeration. Even when
matching densities between the particles and the fluid, cohesive
forces among the particles overcome dispersive forces and
degrade the function of the display. Particle agglomerations
respond less efficiently to the applied field, thereby increasing
the switching time. The agglomeration results in a higher
vulnerability to the action of gravity (limiting the usefulness
of the display in arbitrary orientations). For example, if
a display is oriented vertically, gravity can overcome the
adhesion of a particle to the cell wall and cause the agglomerations
to settle. It is necessary to improve the electrophoretic
display’s stability.
Description Using microencapsulation of electrophoretic
components to improve the electrophoretic display’s stability
is proposed. An electrophoretic display has a two-dimensional
system. This system consists of a layer of transparent microcapsules
that contain an electrophoretic composition of a dielectric
liquid and a suspension of particles. These suspended particles
visually contrast with the dielectric liquid and exhibit surface
charges. The microcapsule layer is located between two visually
transparent electrodes across which a potential difference
may be applied. The potential difference applied across the
two electrodes causes the particles within a capsule to migrate
toward one of the electrodes. The microcapsules function in
a manner similar to pixels, although they are not individually
addressable. When the potential difference is removed, the
particles agglomerate only within one microcapsule. The agglomeration
effect is thus confined to a very small area that is sufficiently
small to be individually unnoticeable. The effect of agglomeration
on the function of the display in general becomes insignificant.
Consequently, microencapsulation eliminates large-scale effects
of agglomeration on an image. This improves the stability
of the display. Therefore, microencapsulation eliminates the
effects of agglomeration and improves the electrophoretic
display’s stability.
Additional information The amount of agglomerating particles is limited
by one microcapsule, so the effects of diminished field responsiveness
in the display and vulnerability to gravity are limited by
the same amount of particles. The dimensions of the microcapsules
range from 5 to 500 microns, and ideally from 25 to 250 microns.
